Fuel efficiency and emission reductions are a concern in the design and operation of an internal combustion engine, such as, for example, a diesel, gasoline, or natural gas engine. Accordingly, a significant amount of research and development work is being directed towards reducing the emissions while maintaining or improving the fuel efficiency of these types of engines. Any increase in fuel efficiency will directly translate to a reduction in the fuel costs associated with operating the engine along with the production of carbon dioxide.
Oxides of nitrogen (“NOx”) are another constituent of engine emissions that researchers are trying to reduce. NOx production is generally proportional to temperatures of combustion and volume of excess air. Unfortunately, fuel efficiency also is generally proportional to these same factors. Conventional NOx reduction techniques include increasing the mass of inert matter, such as water or recirculated exhaust gas, in a combustion chamber prior to combusting a fuel air mixture. These measures may reduce the temperature of combustion and may also reduce the fuel efficiency.
One possible approach to improving fuel efficiency involves improving control over the flow of gases into and out of the engine. This may be accomplished by modifying the typical engine valve actuation system to provide flexibility in the actuation timing of the intake and exhaust valves. This may allow the flow of gases to and from the engine to be tailored to meet the particular operating conditions of the engine.
The engine valves in an internal combustion engine are typically driven by a cam arrangement that is operatively connected to the crankshaft of the engine. The rotation of the crankshaft results in a corresponding rotation of a cam that drives one or more cam followers. The movement of the cam followers results in the actuation of the engine valves. The shape of the cam governs the timing and duration of the valve actuation.
An engine may, however, include a variable valve actuation system, such as described in U.S. Pat. No. 6,237,551 to Macor et al., issued on May 29, 2001. In this type of system, the cam arrangement is configured to hold the engine valves open for a certain period of time and an auxiliary valve is included to selectively disengage the cam assembly. This allows the engine valves to be closed earlier than provided by the timing of the cam assembly and improves the control over valve actuation timing.
The improved control provided by a variable valve actuation system may allow for gains in fuel efficiency. The variable valve actuation system may be operated to selectively implement a variation on the typical diesel or Otto cycle during the operation of the engine. For example, the intake valves may be controlled to implement a “late intake” type Miller cycle. In a late intake Miller cycle, the intake valves are opened for the intake stroke and held open for a portion of the compression stroke of the piston.
The implementation of such an actuation timing variation may, however, have a detrimental effect on the performance of the engine under certain operating conditions. For example, the implementation of a late intake Miller cycle may reduce the compression ratio within the combustion chambers and reduce the amount of air flow through the engine. The reduced compression ratio and air flow may negatively impact the performance of the engine when the engine is subject to a load increase, such as, for example, an acceleration.
The system and method of the present invention solves one or more of the problems set forth above.